Mixed-Berry Oatmeal Crisps

• 2 12-ounce packages frozen mixed berries
• 2 tablespoons cornstarch
• 2/3 cup (packed) golden brown sugar
• 1/2 cup unbleached all purpose flour
• 1/2 cup old-fashioned oats
• 1/2 cup finely chopped walnuts or pecans
• 1/2 teaspoon ground cinnamon
• 6 tablespoons (3/4 stick) chilled unsalted butter, cut into 1/2-inch cubes
• Vanilla ice cream